In an image forming apparatus such as a copying machine or a laser beam printer, a method of fixing an unfixed toner image by contact heating by a heated roller system has heretofore been often used as a method for fixing the unfixed toner image transferred to an image-supporting medium such as plain paper after development with a toner.
However, the fixing device of the heated, roller system; involves a problem that it takes a long time to neat a roller to a finable temperature, and a lot of thermal energy is required, and so a fixing system by a heated film, has been mainly adopted in recent years from the viewpoints of shortening of a time (warming-up time) from turning on the power to a start in copying and energy saving.
In a fixing device of this fixing system by the heated film, a seamless fixing belt obtained by laminating a parting layer formed of a fluorocarbonresin or the like on an external surface of a heat-resistant film formed of polyimide or the like is used.
In the fixing device of such a fixing system by the heated film, the heat-resistant film is heated by, for example, a ceramic heater, and a toner image is fixed by the surface of the heat-resistant film, so that the thermal conductivity of the heat-resistant film is important. However, when the thickness of the neat-resistant film has been thinned, to improve the thermal conductivity thereof, it has been difficult to adopt it in a medium-speed or high-speed machine, because the mechanical strength thereof is lowered and so the belt is difficult to be rotationally moved, and there has been a problem that the ceramic heater or the like is liable to be broken.
In order to solve such a problem, in recent years, there has been proposed a fixing device of a system that a resistance heating layer having a heating element is incorporated into a fixing belt itself, and the fixing belt is directly heated by supplying electric power to this resistance heating layer to fix a toner image (see, for example. Patent Literatures 1 to 4). It may be said that an image forming apparatus in which the fixing device of this system is installed is short in warming-up time, smaller in electric energy consumption than that of fixing system by the heated film, and excellent from the viewpoints of energy saving and high-speed printing.
The fixing belt (heating fixing belt) provided with this resistance heating layer includes a belt that a metallic electrode is embedded in a conductive paste for forming a resistance heating layer and a belt that a metallic tape is stuck on a conductive layer. However, both belts involve a problem that long-term durability is lacking.